With 400 tracks, one backup takes about 40-50 secs and uses about 30% of CPU. phred said it takes 30-45 mins to backup his 18000 tracks. Probably he gets the similar CPU usage during that time. This means if he sets up to auto-backup every hour, this plugin alone consumes 30% of CPU all the time.
Track tag history command too takes lots of resources for comparing tags out of multiple backup files. Sometimes I get "Not Responding" error message while it's loading. But that happened only with 4 backup files. What would happen with 40 or 400 backups?
I don't think managing background priority can resolve this issue. It needs to use far less CPU with much more efficient algorithm, but I'm not sure it's possible.